Temperature-Dependent Structural Characteristics and Guest Occupation Behavior of CO2-Loaded Hydroquinone Clathrates

Hydroquinone (HQ) clathrates prepared by a gas-phase synthesis between HQ host molecules and CO2gas at 4 MPa were characterized via X-ray diffraction (XRD), Raman spectroscopy, solid-state 13C NMR, and scanning electron microscopy. The spectroscopic and morphological results show that pure α-form HQ (α-HQ) was transformed into β-form HQ (β-HQ) clathrate compounds. In particular, we can characterize the occupation behavior of CO2molecules trapped in the HQ clathrate cages via Raman and solid-state 13C NMR spectroscopy. The Raman bands at 1272 and 1378 cm–1for 12CO2and 1364 cm–1for 13CO2and the solid-state 13C cross-polarization/magic angle spinning (CP/MAS) NMR signals at 125 ppm show clear evidence of CO2molecules trapped in the HQ clathrate frameworks. The solid-state 13C CP/static NMR spectra show powder patterns with the value of chemical shift anisotropy −155 ppm, indicating axially symmetric characteristics of CO2guest molecules trapped in the cages of HQ clathrates. To investigate the structural stability and guest release of the CO2-loaded HQ clathrates, temperature-dependent XRD, Raman, and solid-state 13C CP/MAS and CP/static NMR experiments were conducted and compared. Abrupt changes observed near 380 K indicate the temperature-induced structural transition of β-HQ clathrate to α-HQ. In the range from 300 to 380 K, CO2molecules were steadily released from the cages of β-HQ clathrate, and then disappeared entirely above 380 K. From formation kinetics experiments, we confirmed that the reactivity of CO2to form β-HQ clathrates is faster than that of N2and CH4.